The expression of Epstein-Barr virus (EBV)-encoded, growth-transformation-associated proteins was studied in lymphoproliferations of 9 allogeneic bone-marrow transplant (BMT) recipients. lmmunoblots of cell lysates were probed with polyspecific and monospecific antisera directed against EBNA I, 2, 3 and 6, and the membrane protein LMP. All tumors expressed EBNA I and LMP. EBNA 2 was detected in the tumors of 8 patients, and EBNA 3 and 6 in the tumors of 5 patients. The LMP regulatory sequences, 5' of the LMP gene, were totally unmethylated in all 7 cases, while the coding sequences of LMP and EBNA 2 were more methylated in CpG dinucleotides. EBV-transformed lymphoblastoid cell lines (LCL) express EBNA I to 6 and LMP; in contrast, Burkitt lymphomas express only EBNA I. In vitro experiments have shown that EBNA 2, 3 and LMP can generate targets for cytotoxic T cells (CTL). These combined observations are consistent with the hypothesis that the EBV-associated lymphoprofiferative disease of the SMT recipients escapes CTLmediated rejection due to the failure of host immunosurveillance rather than to the down-regulation of immunogenic EBV-encoded antigens.